1. Field of the Invention
The present invention relates to a coplanar waveguide directional coupler which may be advantageously incorporated into flip-chip microwave monolithic integrated circuit (MMIC) arrangements.
2. Description of the Related Art
A directional coupler to which the present invention relates, also known as a "hybrid", is a four port junction device. In an ideal directional coupler, a signal applied to one of the ports is coupled to two of the other ports with a desired coupling ratio, but no part of the signal is coupled to the fourth port. Directional couplers may alternatively be connected to function as RF signal splitters, power combiners, or balanced mixers.
Coplanar waveguide transmission lines are desirable for the interconnection of component elements in microwave assemblies due to their easy adaptation to external shunt element connections as well as to monolithic integrated circuits fabricated on semi-insulating substrates. A coplanar waveguide directional coupler was proposed by Cheng P. Wen, one of the present inventors, in an article entitled "Coplanar Waveguide Directional Couplers", in IEEE Transactions on Microwave Theory and Techniques, June 1970, pp. 318-322. The proposed directional coupler includes two closely spaced signal conductor striplines, and two ground planes disposed on the opposite sides of the striplines. Although suitable for operation at relatively low RF frequencies, the circuit dimensions required to achieve tight coupling for a 3dB (quadrature) coupling at microwave frequencies (10.6 GHz or higher) are beyond the practical limits of microwave integrated circuit fabrication technology.
In the coplanar waveguide directional coupler discussed above, a coupling coefficient K is defined as EQU K=(Zoe-Zoo)/(Zoe+Zoo)
where Zoe and Zoo are the even- and odd-mode impedances of the transmission lines. The directional coupler will operate with minimum reflection if the four ports are matched with an impedance Zo=Zoe.times.Zoo. For the case of a 3dB coupler, K.sup.2 =1/2, and the even- and odd-mode impedances are 120.71 ohms and 20.71 ohms respectively. The gap between two 20 micrometer wide parallel metallic striplines on a substrate having a dielectric constant of 13 must be approximately one micrometer to achieve the desired coupling. This narrow gap requirement over the length of a directional coupler (approximately one quarter of the anticipated signal wavelength) is beyond the existing fine line lithographic capabilities in a current manufacturing environment.
Another type of directional coupler is generally known in the art as a "Lange coupler", and is described in an article entitled "Interdigitated Stripline Quadrature Hybrid", by Julius Lange, in IEEE Transactions on Microwave Theory and Techniques, Dec. 1969, pp. 1150-1151. The Lange coupler includes three or more parallel striplines with alternate lines tied together.
The conventional Lange coupler is not suitable for coplanar waveguide based MMIC fabrication, especially in the flip-chip configuration in which all of the electronic elements and coplanar transmission lines on the MMIC chips face a surface of a substrate on which all of the corresponding coplanar wave transmission lines are formed. This is because the conventional Lange coupler is a microstrip based design, with a single ground plane formed on the opposite surface of the substrate from the signal carrying microstrip lines. Microstrip arrangements are generally undesirable in that the numerous vias which must be formed through the chips and substrate for ground plane interconnection produce fragile MMIC chips.